Under circumstances where it is necessary to provide a continuous indication of the concentration of one component of a sample gas stream, it has long been the practice to use detection techniques that are responsive to the mass rate of flow of the component of interest. In analyzers of the chemiluminescent type, for example, the concentration of the component of interest is determined by measuring the intensity of the light emitted by a mixture of the sample gas stream with a substance that is photochemically reactive with the component of interest. In such instruments the intensity of the light emitted at each instant is dependent upon the total number of photochemical events that are occurring within the detection chamber at that instant and, therefore, upon the mass rate of flow of the component of interest. The latter rate is, in turn, dependent upon the overall or bulk mass rate of flow of the sample gas stream.
Similarly, in instruments of the flame-ionization type, a sample gas-fuel mixture is burned in such a way that the component of interest produces ionized particles in the region between a pair of strongly biased electrodes. The concentration of the component of interest at each instant is then inferred from the instantaneous magnitude of the current flow in an external circuit that connects the electrodes. As a result, the measured quantity of the component of interest at each instant is dependent upon the number of charged particles that are present in the detection chamber at each instant and, therefore, upon the mass flow rate of the component of interest. The latter rate is, in turn, dependent upon the bulk mass rate of flow of the sample gas stream.
In spite of the fact that the detected quantity of a component of interest is dependent on the bulk mass rate of flow of the sample stream that includes it, it has long been the practice to display the results of the measurement process in terms of the concentration of the component of interest, commonly expressed in parts per million. This choice of concentration as an output variable creates a potential source of error, however, since concentration is a quantity that is not dependent upon the bulk mass rate of flow of the sample gas stream, concentration being merely a measure of the relative number of molecules of each species in the sample stream.
In order to prevent real changes in the concentration of the component of interest from being confused with the apparent changes in concentration that result from changes in the bulk mass flow rate of the sample stream, it has been the practice to take steps to fix the rate of flow of the sample stream at a suitable constant value. The most commonly used method for accomplishing this involves the use of a pressure regulator for regulating the pressure of the sample gas stream and the connection of a flow restrictor such as a sample capillary between the pressure regulator and the detection chamber. Together these elements operate (in a manner analogous to that in which a regulated voltage source in series with a fixed resistance provides a fixed current in an external circuit of lower resistance) to provide a constant rate of flow of sample gas through the instrument.
The problem with the above described flow regulation scheme is that, strictly speaking, the pressure drop or difference in pressure across the sample capillary is proportional not to the bulk mass rate of flow of the sample stream therethrough, but rather to the product of the bulk mass rate of flow and the bulk kinematic viscosity of the gas in the sample stream. As a result, even with a constant pressure difference across the capillary, any changes in the bulk viscosity of the sample gas from the value existing at the time the instrument is calibrated (e.g. changes in viscosity that result from changes in the composition of the sample gas stream), result in changes in the mass rate of flow of the sample stream. As a result, since the calibration of the instrument's output display presupposes a constant bulk mass flow rate, there occurs a discrepancy between the actual concentration value and the displayed concentration value. While such discrepancies are typically on the order of a few percent, they can be of great importance in applications such as auto emission analysis in which small differences in pollutant concentration can make the difference between meeting or not meeting state and federal emission standards.